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SC BIOL 460 - Excitation/Contraction Coupling

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BIOL 460 1st Edition Lecture 16 Outline of Last Lecture I. Excitation of Muscle CellsII. Length/Tension RelationshipIII. Motor UnitOutline of Current Lecture I. Muscle Twitch II. Skeletal Muscle Fiber TypesIII. Cardiac MuscleIV. Smooth Muscle Current LectureMuscle Twitch1. Amplitude of twitch is measure of force of contraction2. Result of recruitment3. Summationa. Causes even stronger contraction than possible with single stimulusb. Series elastic elements – some of force of twitch is applied to stretch connective tissue, some total force is lostc. 2nd stimulus occurs when already stretched, all force applied to transducer/bone4. Sustained contraction (tetanus)a. No relaxationb. Some relaxation – incomplete tetanusc. Not how muscles in vivo sustain contraction5. Asynchronous activation of motor unitsa. Each motor unit contracts in a twitch like fashionb. Doesn’t contract at the same timeSkeletal muscle fiber types1. Classify based on contraction speed and ATP production2. Contraction speed based on ATPase ability of myosin cross bridges3. Some achieve maximum tension very quickly, some much slower4. HumansThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.a. Fast twitch – 7.5millisecondsb. Slow twitch – 100 milliseconds5. Can classify based on principle mechanism of ATP synthesisa. Red fibersi. Oxidativeii. Most ATP generated through redox reactionsiii. Red because well vascularizediv. Have a lot of myoglobinv. Many mitochondriab. White fiersi. Glycolyticii. Produce ATP through glycolysisiii. Fermentationiv. Final e- acceptor is pyruvatev. High concentration of glycolytic enzymesvi. Larger in diameter6. Classificationsa. Slow oxidative/slow twitch fatigue-resistanti. Postural musclesii. Smallest motor unitsb. Fast glycolytic/fast twitch fatigueablei. Biggest motor unitsc. Fast oxidative/fast twitch fatigue-resistanti. Medium motor unitsCardiac Muscle 1. In heart2. Striated – typical sarcomeres3. Contract with sliding filament mechanism4. Innervated by visceral motor neurons5. Short, uninucleate, branched cells6. Attached by gap junctions7. Intercalated disc – discs between cardiac muscle cells8. 1 AP is quickly spread through all cells in myocardium9. Myocardium – mass of cardiac muscle cellsa. Atrial myocardiumb. Ventricular myocardium10. Myocardium connected via intercalated discs11. No motor Units12. Automaticity – contracts on its own (AP changes resting frequency)Smooth Muscle1. Innervated by autonomic neurons2. Two typesa. Multiunit – iris of eye; individual smooth muscle cells all receiving innervation by one axonb. Single Uniti. Much more commonii. Walls of hollow organsiii. Innervation by synapses en passantiv. Cells are connected together with gap junctionsv. All contract togethervi. Generate peristaltic waves3. Non-striated – no A and I bands, no sarcomeres4. Cells are spindle shaped, uninucleate5. No z discs, thin filaments attach to sarcolemma or masses of protein called dense bodies6. Thick myofilaments are less common (much more actin than myosin, 16:1 ratio)7. Thick myofilaments are composed of myosin organized with cross bridges parallel to shaft (unlike skeletal muscle)8. No bare zone without cross bridges9. LOOK AT FIGURE 12.3710. Single unit SM – innervation occurs via gap junctions11. Smooth musclea. More thin: thick ratio (16:1) than skeletal muscle (2:1)b. No bare zone on thick myofilamentc. Cross bridges parallel to myosin bandd. Makes smooth muscle better able to stretche. Slow, sustained contractions (slow myosin ATPase’s)12. Excitation/Contraction coupling in Smooth musclea. Trigger is Ca2+ from extracellular environment (NOT FROM SR)b. Sarcolemma contains Ca2+ VGCc. EPSPs/Graded contractions – result of amount of Ca2+ going in celli. Low Ca2+, weak contractionii. High Ca2+, strong contractiond. No troponin or tropomyosine. Ca2+ in cell, bonds to calmodulin, making Ca2+/calmodulin complexf. Activates myosin light chain kinaseg. Phosphorylates myosin light chainsh. Cross bridges attach to thin myofilaments and cross bridge cyclei. Turning contraction ofi. Ca2+ pumped out of sarcoplasm via Ca2+ pumpsii. Myosin phosphatase dephosphorylates myosin light


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